Molecular chaperones are a group of highly conserved proteins that facilitate protein folding and assembly but do not become part of the final product. It is known that such proteins facilitate the folding of newly synthesized and denatured monomeric proteins, but it is not clear whether they play a role in the subsequent assembly of subunits into multimeric particles. We are interested in studying the role of such molecular chaperones, as well as other host factors, in the assembly of viral particles. To do this we recently established a cell-free system in which we can synthesize and assemble bona-fide Hepatitis B Virus (HBV) capsids, which are composed of 180 HBV core polypeptide monomers (Lingappa et al., 1994, J. Cell Biol. in press). Production of unassembled core polypeptide monomers can be dissociated from capsid assembly in this system. Capsid assembly proceeds by way of a high molecular weight intermediate. TCP-l, a cytosolic chaperonin of eukaryotic cytosol, is associated with core polypeptides in the intermediate, but not with core polypeptides in either the unassembled starting material or in the capsid product. Furthermore, upon isolation of the assembly intermediate, different manipulations of energy substrates result in the release of either completed capsids, or of unassembled core polypeptides. These data suggest that a chaperonin of eukaryotic cytosol may play a role in the assembly of viral particles. We propose here l) to characterize the components of the assembly intermediate that we have found, 2) to extend this system to the study of HBV capsid envelopment, and 3) to adapt our cell-free system for assembly of another viral capsid, that of human immunodeficiency virus (HIV). These approaches to understanding viral assembly hold promise for clinically relevant anti-viral strategies in the future.